This invention relates to endoluminal prostheses such as vascular repair devices, and in particular intravascular stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel or coronary artery, to maintain the lumen""s patency. Stents are particularly useful in the treatment of atherosclerotic stenosis and are most frequently used in connection with coronary angioplasty.
Stents are tubular, usually cylindrical devices which hold open a segment of blood vessel or other body lumen. They also are suitable to support and hold back a dissected arterial lining that can occlude the lumen. At present, numerous models of stents are marketed throughout the world. While some of these stents are flexible and have the appropriate strength and rigidity needed to hold open a lumen such as a coronary artery, each stent design typically represents a compromise between the stent""s flexibility and its radial strength. What has been needed, and heretofore unavailable, is a stent which has a high degree of flexibility so that it can be advanced through tortuous lumen and readily expanded, and yet have the mechanical strength to hold open the lumen or artery into which it is implanted and provide adequate vessel wall coverage.
At least some in the stent industry also perceive a problem with xe2x80x9cfishscaling.xe2x80x9d Fishscaling, describes the twisting or bending of stent struts, which results in the struts not conforming to a generally cylindrical plane around the circumference of the stent. Fishscaling can result from the manufacturing process, as in the case of the Medinol, Ltd. NIR(copyright) stent. Fishscaling also can occur during the stent placement process, such as when portions of the stent surface are forced outward as the stent bends while advancing through tortuous lumen. Some in the stent art believe that fishscaling can damage the blood vessel through which the stent is being advanced. Therefore, there is a perceived need for a stent that reduces or eliminates fishscaling.
The present invention is directed to an endoluminal prosthesis, such as an intravascular stent, which is highly flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but which is strong and stable enough radially in its expanded condition to maintain the patency of a body lumen when the stent is implanted therein. The stent also reduces fishscaling.
The stent of the present invention includes a plurality of generally cylindrical elements, also known as rings, that are interconnected to form the stent. The stent typically is mounted on a balloon catheter if it is balloon expandable, or else it can be mounted on a catheter without a balloon if it is self-expanding.
Each of the cylindrical rings or elements has a proximal end and a distal end and a cylindrical plane defined by a cylindrical outer wall surface that extends circumferentially between the proximal end and the distal end of the cylindrical ring. In the preferred embodiment, cylindrical rings are interconnected by three links which attach one cylindrical ring to an adjacent cylindrical ring. The links are positioned substantially within the cylindrical plane of the outer wall surface of the cylindrical rings. The design of these highly flexible, interconnected members provides for uniform scaffolding and a high percentage of vessel wall coverage.
The cylindrical rings typically are formed of a plurality of peaks and valleys. A straight strut, also called a bar arm, and a curved, or nonlinear bar arm extend from each peak or valley. In several preferred embodiments, this arrangement gives each ring the appearance of a series of figure-eights. In this configuration, at least one link attaches each cylindrical ring to an adjacent cylindrical ring. In the preferred embodiments, each cylindrical ring has six peaks and valleys and three to six connecting links. The cylindrical rings and flexible links are preferably not separate structures, although they have been conveniently referred to separately for ease of identification.
Typically, a balloon expandable stent is made from a stainless steel alloy or similar material. The cylindrical rings of the stent are plastically deformed when expanded by the balloon.
The links may take various configurations. One such configuration is a straight link. Another is an undulating or serpentine shape, which makes the links more flexible. The undulating links can include bends connected by straight portions, wherein the substantially straight portions are perpendicular to the stent""s longitudinal axis. Another configuration places one or more apertures, such as an oval, rectangle, or dog bone shape, in a straight link. The apertures are typically longer in one direction than another, with the longer direction oriented preferably perpendicular to the longitudinal axis of the stent. These links are described in two United States patent applications by Ainsworth and Cheng, Ser. No. 09/746,746, filed Dec. 22, 2000, and assigned to Advanced Cardiovascular Systems, Inc., Santa Clara, Calif. (ACS) the assignee of the present application, and of Ser. No. 09/564,151, filed May 3, 2000, also assigned to ACS. The entire earlier applications are incorporated herein by reference.
In the case of the undulating links that interconnect the cylindrical rings, the positioning of the unexpanded links also enhances the flexibility by allowing uniform flexibility when the stent is bent in any direction along its longitudinal axis. The cylindrical rings of the stent can expand radially outwardly without a balloon when the stent is formed from a superelastic alloy, such as nickel titanium (NiTi) alloys. These so-called xe2x80x9cself-expandingxe2x80x9d stents expand upon application of a temperature change or when a stress is relieved, as in the case of a pseudo-elastic phase change.
The number of peaks, valleys, links, and cylindrical rings can be varied as the application requires. When using flexible links, the link typically does not expand when the cylindrical rings of the stent expand radially outwardly, but the links do continue to provide flexibility and to also provide a scaffolding function to assist in holding open the artery. Each flexible link is configured so that it promotes flexibility.
The configuration of the rings provides the stent with a high degree of flexibility along the stent axis, and reduces the tendency of stent fishscaling. Further, because the links do not expand or stretch when the stent is radially expanded, the overall length of the stent is substantially the same in the unexpanded and expanded configurations. In other words, the stent will not appreciably shorten upon expansion.
Other embodiments of the invention also include figure-eight type rings with a linear and non-linear bar arm. Ring orientation and link shape vary the features of these other embodiments.
The stent is formed from a tube by laser cutting the pattern of cylindrical rings and flexible links in the tube. The stent also may be formed by laser cutting a flat metal sheet in the pattern of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent.